Asmall construction firm specializes in building and selling single-family homes. the firm offers two basic types of houses, model a and model b. model a houses require 4000 labor hours, 2 tons of stone and 2000 board feet of lumber. model b houses require 1 hours of labor, 3 tons of stone and 2000 board feet of lumber. the firm has currently 4 hours of labor, 150 tons of stone and 2 board feet of lumber. model a yields $1000 profit and model b yields $2000 profit. assume that the firm is able to sell all units it can build. what mix of model a and model b units should the firm construct to maximize the profit?